A squib driver integrated circuit (IC) for driving an airbag squib is disclosed in a data sheet of a squib driver TPD2004F of Toshiba Semiconductor Company published on Sep. 11, 1998.
FIG. 3 is a simplified circuit diagram of the squib driver IC. A high-side circuit 1 includes an n-channel metal oxide semiconductor field effect transistor (MOSFET) 3 and a high-side driver 2, which is a charge-pump type. The high-side driver 2 controls a gate voltage of the MOSFET 3. A drain and a source of the MOSFET 3 are connected to a power supply terminal VBB and an output terminal SH, respectively. A low-side circuit 4 includes an n-channel MOSFET 6 and a low-side driver 5. The low-side driver 5 switches on and off the MOSFET 6. A drain and a source of the MOSFET 6 are connected to an output terminal SL and a ground terminal GND, respectively. A squib 7 (shown as a resistor in FIG. 3), which is a load, is interposed between the output terminals SH, SL.
The MOSFET 6 is kept fully turned on (saturated) during normal operation. The MOSFET 6 is turned off, when the MOSFET 3 cannot control the load current. Thus, the MOSFET 6 is capable of interrupting the current path to the squib 7.
Less voltage is applied between the drain and the source of the MOSFET 6 during normal operation, and consequently heat produced in the MOSFET 6 is negligible. In contrast, the voltage of the power supply terminal VBB is applied almost directly between the drain and the source of the MOSFET 3, and consequently heat produced in the MOSFET 3 is considerable. Even though there are the MOSFET 3 and the MOSFET 6 in the circuit, most of heat is produced in the MOSFET 3. Thermal margin of the MOSFET 3 is reduced accordingly. In other words, the risk of thermal breakdown of the MOSFET 3 is increased. Therefore, it is required to increase the size of the MOSFET 3 to prevent thermal breakdown of the MOSFET 3 and cost competitiveness is weakened.